1. Field of the Invention
The present invention relates to a method and apparatus for forming a resin film, in particular, to a method and apparatus for forming a resin film in which a resin for use in the edge portion of the resin film is applied to both side edges in the crosswise direction of the resin film main body, which is formed of a resin for use in the middle portion of the resin film, at the time the resins in the molten state are extruded through an extruding die to form the resin film.
2. Description of the Related Art
In a resin film having been extruded through an extruding die, its selvages (both side edges in the crosswise direction of the resin film) are usually trimmed off in the post-processing. When the resin film is formed of a resin hard to recycle or poor in thermal stability, the trimmed-off selvages cannot be recycled, which leads to decrease in the yield of the resin film. Hence, in order to make possible the recycling of resin films, there have been proposed a technique for forming a resin film in which a resin for use in the edge portion of the resin film is applied to both side edges in the crosswise direction of the resin film main body (the portion of the resin film left after trimming off its selvages and to be an end product), which is formed of a resin for use in the middle portion of the resin film. For example, when forming the resin film main body of a resin poor in thermal stability, a resin for the edge portions excellent in thermal stability is applied to both side edges of the main body and then the resin for the edge portions is trimmed off as the selvages. This allows the recycling of resin films and the improvement in productivity, because the trimmed-off resin for the edge portions is hard to subject to heat deterioration even when it is used repeatedly.
However, the techniques still have disadvantages in that when the physical properties of the resins for the middle portion and for the edge portions of a resin film are different or the film forming condition such as applying condition is different, the resins are likely to separate from each other or the boundary between them is likely to be in disorder. If the boundary is in disorder, the proportion of the inclusion of the resin for the middle portion to the trimmed-off selvages becomes large, which causes not only the deterioration of the resin film recyclability, but also the decrease in productivity, due to the decrease in the portion as a resin film main body.
As a measure to prevent the occurrence of the above problems, Japanese Patent Application Publication No. 2002-240126 discloses a method to prevent the film separation of the resin for the edge portions and the resin for the middle portion by wrapping up the former in the latter. However, this has the disadvantage in that, since the resin for the edge portions is wrapped up in the resin for the middle portion, the resin for the middle portion accounts for a larger part of the resin of the trimmed-off selvages than the resin for the edge portions, causing the deterioration in not only recyclability but also productivity. In other words, in order to increase the recyclability and productivity of resin films, it is important to arrange the resins for the edge portions and for the middle portion in such a manner as to avoid their mingling as much as possible when trimming off the selvages of the resin film, and furthermore, it is important to make their boundary in order and their boundary line clear. It is necessary to avoid the film separation of the two resins while satisfying the above requirements.
Further, Japanese Patent Application Publication No. 1-64822 discloses an apparatus for forming such kind of resin film, which adopts a detachable assembly. Still further, Japanese Patent Application Publication No. 7-76038 discloses an apparatus for forming such kind of resin film, in which a resin for the edge portions of a resin film and a resin for the middle portion of the resin film are layered in the crosswise direction in an extruding die. However, even with these apparatuses, the problems of the film separation of the resins and the disorder at the boundary between the resins cannot be resolved.